1. Field of the Invention
This invention relates to an automatic focus adjusting device, and particularly to an improvement in the automatic focus adjusting device of a microscope.
2. Description of the Prior Art
In an example of the automatic focus adjusting device of the conventional microscope, first and second one-dimensional image sensors are disposed rearwardly and forwardly, respectively, of the image formation point, namely, at the outer focus position and the inner focus position, respectively. Hereinafter, the outer focus position and the inner focus position will be referred to as the first position and the second position, respectively. The high frequency components of the image outputs from the image sensors become greatest when images are formed on the image sensors. If the high frequency components from the first and second image sensors for movement of a sample stage are symmetrical with respect to the image formation point, the zero crossing point of the difference between the high frequency components of the two outputs may be said to be the image formation point.
That the zero crossing point is the image formation point is based on the premise that the light information entering the first and second image sensors is completely symmetrical with respect to the image formation point. However, in the conventional arrangement and construction, the image at the first position has been larger than the image at the second position and thus, a difference has been created between the sizes of the images on the two sensors. Accordingly, if the lengths of the sensors are equal, a difference has been created between the amounts of light information seized. Also, due to the nature of the objective lens, the enlarged image at the first position has been poorer in contrast and so, the high frequency component output thereof has been relatively small. Accordingly, the outputs of the two sensors have become asymmetric and therefore, the zero crossing point of the difference therebetween has not exactly corresponded to the image formation point, thus making it difficult to effect the focusing with high accuracy.